1. Field of the Invention
This invention pertains to laser writers for exposing patterns on light-sensitive photoresist on semiconductor wafers, printed circuit boards, photomasks, hybrid circuit boards, ceramic boards and the like, and for exposing patterns on light-sensitive film or glass and the like.
2. Description of the Related Art
Many areas of technology are today heavily dependent on the availability of semicustom and custom integrated circuits, such as gate arrays and ASICs (Application Specific Integrated Circuits). Long cycle time in the development of these key components frequently results in components that have not been optimized adequately and that have been very expensive to develop. Extensive computer simulation is required to reduce the number of redesigns. Even with this simulation, system validation requires actual devices for in-circuit testing. Better system performance often can be obtained with less simulation by the early availability of low cost prototype integrated circuit devices.
Effective prototyping of integrated circuit devices requires several key features. Initially, effective design capture is required This capability has been broadly implemented in the industry with tools such as CAD/CAM. The initial data checking and validation tools are in place and broadly used. Standard design flows have been established from this design capture area to the patterning data bases. These flows can usually be executed quickly, often resulting in overnight delivery of the plot data base.
Frequently, long delays occur at this point. The conventional approach is to fabricate and qualify a standard set of production, high volume photomasks at a high cost, and days to weeks of processing, queing, and qualification time. When the actual semiconductor wafers are to have their photoresist exposed, processing capacity is significantly reduced by the downtime required for loading, aligning, and calibrating each new reticle, for example. This can be as quick as one minute or, more commonly, as long as 15 minutes. Thus, the capacity of a critical piece of equipment such as a wafer stepper can drop below four wafers per hour. From the standpoint of assured integrated circuit device completion, it is desirable to write more than one wafer. If only one integrated circuit device design is used per wafer, the result can be building more than 1200 devices for a prototype test run of three wafers. If more than one integrated circuit design is used per wafer, then the reticle loading problems become even more severe. The end result is very expensive, long turnaround prototype integrated circuit device fabrication. The severity of the problem increases as the number of custom features in an integrated circuit device increases. Thus, semi-custom devices such as gate arrays commonly are fabricated notwithstanding these difficulties, but prototyping of fully custom devices is usually restricted to long production runs or to very high value products.
An alternative to photolithographic steppers is direct e-beam patterning, which has been applied in a limited number of cases. E-beam patterning has proven effective where small quantities of very high value devices are being built. An advantage to e-beam machines is that it can be practical to fabricate multiple integrated circuit designs on a single wafer since no physical mask or reticle is required. E-beam machines are, however, very costly and slow, resulting in patterning costs that are not acceptable for production runs, once the prototype devices have been produced. Furthermore, e-beam machines have a poor record of uptime, which further restricts their use for photolithographic writing on wafers. Another disadvantage to e-beam wafer writers is that they require the use of special electron beam sensitive resists. These resists have not shown the chemical durability or freedom from defects known as pinhole defects, obtainable with modern optically sensitive photoresists. Thus, e-beam masks require inspection and frequently require repair to achieve acceptable defect levels. With resist coated wafers, however, this type of repair is not physically practical.
Printed wiring board fabrication has the same types of patterning problems as those of integrated circuit patterns on wafers, but on a different scale. Printed wiring boards may be exposed by writing directly on photoresist covering the surface of a board, using a beam of light or electrons. Alternatively exposure can take place by projecting patterns of light and dark on the surface of a board using a glass plate or film artwork.
In addition to the same problems already mentioned above concerning e-beam use for direct patterning on wafers, e-beams could not be used to produce patterns on printed wiring boards much above six inches by six inches. For printed wiring boards greater than this size, glass plate or film artwork or masks would have to be used. It should be noted, however, that e-beam machines normally are not used to produce printed wiring boards and such use is only hypothetical. The standard printed wiring board mask manufacturing machine has been Gerber, which uses photographic exposure through various apertures to describe the geometries.
Current systems for creating printed wiring board masks suffer from problems of poor fabrication cycle time, storage and maintenance of master artwork over a period of years. Additionally, the artwork must be carefully inspected for errors, adding to the cycle time.
Attempts have been made to produce a laser writer capable of writing directly on photoresist or creating printed wiring board artwork, using rotating mirror technology. Rotating mirror technology has been used to produce laser character printers, however, in the field of artwork generation and direct writing on printed wiring boards, the technology has some serious drawbacks.
Prior art rotating mirror scanner laser writers for printed wiring board applications produce what is basically a circular scan. For these systems to write at uniform velocity, an F-theta lens is required. The field correction provided by this lens is incomplete. If reasonably accurate laser beam positioning on a target is to be achieved, additional levels of position correction are required. Some of these correction techniques include the use of a separate pilot laser position sensing beam monitored by a CCD line scanner, or the equivalent. Other correction methods include the use of galvanometer mirror systems to apply corrections. All of these methods of correction result in incomplete correction that limits the accuracy and resolution of these systems.
Another source of errors inherent in the use of rotating mirror systems is the multifaceted mirror assembly itself. Small mirror assemblies are limited in the number of reflecting sides that can be ground on the assembly, and still maintain the necessary accuracy.
Larger mirror assemblies may be used to correct the problems inherent in small mirror assemblies, however, larger rotating mirror assemblies suffer from distortion caused by forces generated by the necessary large rotational velocities.
Mirror assemblies also suffer from problems of balance and the support bearing performance.
As a consequence of the problems inherent in rotating mirror writer systems, the systems are limited in writing speed, accuracy and stability by the mirror assembly problems mentioned above.
Another attempt to produce a laser writing system uses an acousto-optical Bragg cell as a laser beam deflector. This system is disclosed in U.S. Pat. Nos. 4,541,712 and 4,620,288. What is disclosed is a laser-based pattern generating system for writing patterns on light-sensitive material to produce reticles and for direct on-wafer pattern generation. This system uses a beam splitter to split a laser beam into an array of sixteen parallel beams, which is swept by the Bragg cell from side-to-side in raster-scan fashion to form swaths of sixteen parallel rows of modulated-light-exposed strips on the surface of a target such as a reticle or wafer. The swaths are laid down in parallel rows contiguous to one another to build up, swath-at-a-time, a desired image on the target surface.
After the 16 laser beams pass through the beam splitter they are individually modulated using acousto-optical modulators. The beams are then recombined in an attempt to form an array of precisely spaced, parallel beams which must strike the surface of the target in a precisely spaced pattern. This is required to achieve accurately parallel lines and to prevent wave interference effects between the beams.
After the 16 beams are recombined they are deflected by the Bragg cell reflector in raster scan fashion to sweep across the surface of the target. The acousto-optical bragg cell for deflecting the 16 recombined beams has a sweep rate dependent upon the frequency slew rate of the oscillator producing acoustic waves travelling transversely to the optical axis in the deflector. Linearity of slew rate is very difficult to achieve, which means that linearity in the sweep of the laser array is impossible to achieve for all practical purposes.